This invention relates to digital signal processing for synthetic aperture radar (SAR), and more particularly to pipelined digital signal processing for producing real-time high-resolution SAR images using a hybrid processor comprised of a fast Fourier transform (FFT) for correlation in the SAR azimuth (along-track) direction and a time-domain transversal filter to accommodate the dispersion of the SAR response in the range direction. This dispersion is caused by the variation of target distance to the SAR transmitter-receiver over the SAR aperture length, and is often referred to as the range migration effect that must somehow be compensated in the SAR data processing.
The utility of synthetic aperture radar, under investigation for approximately three decades, has been found to be a useful instrument for a number of geological and oceanographical applications. The launch of a spaceborne SAR on the SEASAT satallite in June, 1978, marked the advance of SAR remote sensing technology from a conventional airborne environment into an earth orbiting spaceborne environment.
Two important aspects of SAR systems are the high data acquisition rate and the complicated data reduction process. Real-time data reduction to form images for the airborne SAR system was accomplished in the mid 1970's. Real-time spaceborne SAR processing for producing high quality images is still a continuing challenge. Much of the operational SEASAT SAR image formation process is currently being handled by an optical processor and a moderate throughput software-based digital SAR processing system. In order to meet the objectives of potential candidates of future operationally oriented spaceborne SAR missions in which the image data will be used in a prompt fashion to derive surface state of temporal change data, a real-time or near real-time SAR processing capability is required.
Techniques for achieving this real-time processing to form SAR images involve a number of engineering disciplines. A survey of the state-of-the-art SAR processing indicates the following conclusions: (1) the theoretical framework for SAR sensor operation and data processing is reasonably well understood; (2) many techniques for SAR data reduction for forming images have been devised, but their performance characteristics and design tradeoffs have not been fully established; and (3) the major cost factor of a high quality real-time SAR processor is the need for a tremendous number of high speed electronic devices and associated interconnective networks.
Existing candidate architectures for a real-time SAR processor include the time-domain transversal filter and the frequency-domain multistage Fast Fourier Transform (FFT). A comparison of these two types of approaches yields differences in the arithmetic computation and processing control requirements.
In digital processing of SAR data, the raw data that has been quadrature demodulated and digitized is first range compressed. This range compression is required only if the radar employs a coded waveform in its transmitted pulses. The output of this range correlation process is sequential in the range dimension. Therefore, before starting a two-dimensional azimuth correlation, one needs to perform the so-called corner-turning or matrix transpose on the range correlated data. This is a quite time-consuming process. If two-dimensional azimuth correlation is done entirely in the time domain, then fine interpolation between data at discrete locations must be performed to trace the maximum amplitude response. Also the azimuth dimension is usually quite large, so a large time-domain transversal filter is required to perform the azimuth correlation. All of these considerations make the time-domain approach undesirable.
But the alternative to perform a two-dimensional azimuth correlation entirely in the frequency domain is also undesirable. For severe range migration, this alternative requires a large amount of hardware to do the two-dimensional transform. Furthermore, this alternative requires the transform of the two-dimensional reference function. Hence changing the reference function is not easy.
What is required is a digital synthetic aperture radar (SAR) azimuth correlator which produces real-time, high-resolution SAR imagery utilizing the frequency domain for range and azimuth correlation without requiring excessively large memory capacity for the SAR data being processed, and a corner-turn technique that will alleviate the complicated matrix transpose problem in a two-dimensional azimuth correlation following the range correlation with spectral filtering performed in the azimuth dimension, and a range transversal filtering in the time domain to conveniently accommodate the range migration effect. These features yield on efficient and modular SAR processor with which an exact SAR correlation algorithm may be implemented.